Prune path prunes above zipper root

```rs
fn prune_crosses_zipper_root() {
    let mut map = PathMap::<u64>::new();
    map.create_path(b"ab");
    let pruned = map.write_zipper_at_path(b"ab").prune_path();
    assert!(map.path_exists_at(b"ab"));
    assert_eq!(pruned, 0);
}
```